Synthesis of glycols via transfer hydrogenation of alpha-functional esters with alcohols

ABSTRACT

A transfer hydrogenation process for forming vicinal diols by hydrogenating 1,2-dioxygenated organic compounds using alcohols as the reducing agent instead of the traditional H2 gas. The transfer hydrogenation is carried out under milder conditions of temperature and pressure than is typical for ester hydrogenation with H2. The milder conditions of operation provide benefits, such as lower operating and capital costs for industrial scale production as well as savings in product purification due to the avoidance of by-products from exposure of reaction mixtures and products to high temperatures.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application62/540,354 filed on Aug. 2, 2017 under 35 U.S.C. § 119(e)(1), the entirecontent of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to the field of organic chemistry. Itparticularly relates to the catalytic transfer of hydrogen from alcoholsto α-functional esters to form 1,2-diols.

BACKGROUND OF THE INVENTION

Conventional processes for preparing ethylene glycol (EG) and propyleneglycol (PG) entail partial oxidation of ethylene or propylene followedby hydration of the resulting epoxides. More recently, hydrogenation ofglycolate or oxalate esters has been proposed as alternative methods forpreparing EG from alternative feedstock materials. These latter methods,however, suffer from one or more drawbacks, such as requiring the use ofexpensive precious or rare metal catalysts, high temperatures, and/orhigh hydrogen pressures.

Thus, there is a need in the art for a process for hydrogenatingα-functional esters (such as glycolate esters, lactate esters, andoxalate esters) that does not suffer from these drawbacks.

The present invention addresses this need as well as others, which willbecome apparent from the following description and the appended claims.

SUMMARY OF THE INVENTION

The invention is as set forth in the appended claims.

Briefly, the invention provides a process for preparing a 1,2-diol. Theprocess comprises contacting an ester of the formulas (IV) or (V):

with an anhydrous C₂-C₁₂ alcohol in the presence of a catalyst of theformula (I):

in a reactor at ambient pressure and elevated temperature for a timesufficient to form a 1,2-diol,wherein

R¹ and R² are each independently an alkyl, aryl, alkoxy, aryloxy,dialkylamido, diarylamido, or alkylarylamido group having 1 to 12 carbonatoms;

R³ and R⁴ are each independently an alkyl or aryl group having 1 to 12carbon atoms, if E is nitrogen;

R³ and R⁴ are each independently an alkyl, aryl, alkoxy, aryloxy,dialkylamido, diarylamido, or alkylarylamido group having 1 to 12 carbonatoms, if E is phosphorus;

R¹, R², and P may be connected to form a 5 or 6-membered heterocyclicring;

R³, R⁴, and E may be connected to form a 5 or 6-membered heterocyclicring;

R⁵ and R⁶ are each independently a C₁-C₆ alkylene or arylene group;

E is phosphorus or nitrogen;

L is a neutral ligand;

R¹⁰ is an alkyl or aryl group having 1 to 20 carbon atoms;

R¹¹ is each independently hydrogen, or an alkyl or aryl group having 1to 20 carbon atoms; and

R¹² is hydrogen, or an alkyl, aryl, alkoxy, or aryloxy group having 1 to20 carbon atoms.

DETAILED DESCRIPTION OF THE INVENTION

It has been surprisingly discovered that certain iron-based catalystsare effective for the transfer hydrogenation of α-functional esters(such as glycolates or oxalates) to 1,2-diols in the presence of analcohol as a sacrificial donor. The transfer hydrogenation (TH) uses asacrificial alcohol (RR′CHOH) donor molecule instead of H2 gas as thereducing agent. Since no additional H2 pressure is required, thesereactions can be run under ambient (or near ambient) pressure and atmild temperatures (e.g., ˜100° C.).

Thus, the present invention provides a process for preparing a 1,2-diol.The process comprises contacting an ester of the formulas (IV) or (V):

with an anhydrous C₂-C₁₂ alcohol in the presence of a catalyst of theformula (I):

in a reactor at ambient pressure and elevated temperature for a timesufficient to form a 1,2-diol.

R1 and R2 in the formula (I) are each independently an alkyl, aryl,alkoxy, aryloxy, dialkylamido, diarylamido, or alkylarylamido grouphaving 1 to 12 carbon atoms.

R3 and R4 in the formula (I) are each independently an alkyl or arylgroup having 1 to 12 carbon atoms, if E is nitrogen.

R3 and R4 in the formula (I) are each independently an alkyl, aryl,alkoxy, aryloxy, dialkylamido, diarylamido, or alkylarylamido grouphaving 1 to 12 carbon atoms, if E is phosphorus.

R5 and R6 in the formula (I) are each independently a C1-C6 alkylene orarylene group.

E in the formula (I) is phosphorus or nitrogen.

L in the formula (I) is a neutral ligand.

R1, R2, and P in the formula (I) may be connected to form a 5 or6-membered heterocyclic ring.

R3, R4, and E in the formula (I) may be connected to form a 5 or6-membered heterocyclic ring.

R10 in the formulas (IV) or (V) is an alkyl or aryl group having 1 to 20carbon atoms.

R11 in the formula (IV) is each independently hydrogen, or an alkyl oraryl group having 1 to 20 carbon atoms.

R12 in the formula (V) is hydrogen, or an alkyl, aryl, alkoxy, oraryloxy group having 1 to 20 carbon atoms.

One or more of R1, R2, R3, and R4 may be substituted with one or moregroups selected from ethers and amides. The substituents on R1, R2, R3,and R4, if any, may be the same or different.

Examples of ether groups include methoxy, ethoxy, isopropoxy, and thelike.

Examples of amide groups include dimethylamido, diethylamido,diisopropylamido, and the like.

As used herein, the term “alkyl” refers to straight, branched, or cyclicalkyl groups. Examples of such groups include methyl, ethyl, n-propyl,isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl,tert-pentyl, neopentyl, isopentyl, sec-pentyl, 3-pentyl, cyclopentyl,n-hexyl, isohexyl, cyclohexyl, and the like.

The term “aryl” generally refers to phenyl or naphthyl. But inconnection with R10 and R11, the term “aryl” includes not only phenyland naphthyl, but also other hydrocarbon rings containing alternatingsingle and double bonds, such as indene, acenaphthylene, anthracene,phenanthrene, tryphenylene, pyrene, etc.

The term “alkylene” refers to a divalent alkyl group.

The term “arylene” refers to a divalent aryl group.

The term “alkoxy” refers to an —OR group, such as —OCH3, —OEt, —OiPr,—OBu, —OiBu, and the like.

The term “aryloxy” refers to an —OAr group, such as —OPh, —O(substitutedPh), —Onaphthyl, and the like.

The term “dialkylamido” refers to an —NR′R″ group, such asdimethylamido, diethylamido, diisopropylamido, and the like.

The term “diarylamido” refers to an —NAr′Ar″ group, such asdiphenylamido.

The term “alkylarylamido” refers to an —NRAr group, such asmethylphenylamido.

The term “neutral ligand” refers to a ligand with a neutral charge.Examples of neutral ligands include carbon monoxide, an ether compound,a phosphine compound, an amine compound, an amide compound, a nitrilecompound, and an N-containing heterocyclic compound. Examples of neutralphosphine ligands include trimethylphosphine, tricyclohexylphosphine,triphenylphosphine, and the like. Examples of neutral amine ligandsinclude trialkylamines, alkylarylamines, and dialkylarylamines, such astrimethylamine and N,N-dimethylanaline. Examples of neutral nitrileligands include acetonitrile. Examples of neutral N-containingheterocyclic ligands include pyridine and 1,3-dialkyl- ordiaryl-imidazole carbenes.

In one embodiment, R1, R2, R3, and R4 are all isopropyl. In anotherembodiment, R1, R2, R3, and R4 are all phenyl.

In one embodiment, R5 and R6 are both —(CH2CH2)-.

In one embodiment, E is phosphorus.

In various embodiments, the catalyst of the formula (I) has the formula(1c):

where ^(i)Pr represents an isopropyl group.

The ester of the formulas (IV) or (V) useful in the present invention isnot particularly limiting. In various embodiments, the ester includes acompound of the formula (IV). Such α-hydroxy carboxylic acid esters aresometimes referred to as glycolates. In various other embodiments, theester includes a compound of the formula (V) where R12 is hydrogen, oran alkyl or aryl group. Such α-carbonyl carboxylic acid esters aresometimes referred to as glyoxalates. In yet various other embodiments,the ester includes a compound of the formula (V) where R12 is an alkoxyor aryloxy group. Such 1,2-diesters are sometimes referred to asoxalates.

Examples of the glycolates of the formula (IV) include methyl glycolate,ethyl glycolate, propyl glycolate, isopropyl glycolate, butyl glycolate,isobutyl glycolate, sec-butyl glycolate, 2-ethylhexyl glycolate,tert-butyl glycolate, cyclohexyl glycolate, phenyl glycolate, benzylglycolate, naphthyl glycolate, methyl lactate, ethyl lactate, propyllactate, isopropyl lactate, butyl lactate, isobutyl lactate, sec-butyllactate, 2-ethylhexyl lactate, tert-butyl lactate, cyclohexyl lactate,benzyl lactate, methyl □-hydroxybutyrate, methyl □-hydroxyvalerate,methyl □-hydroxy-4-methylvalerate, methyl □-hydroxycaproate, methyl□-hydroxy-3-methylbutyrate, methyl □-hydroxy-3-methylvalerate, methyl□-hydroxy-3,3-dimethylbutyrate, methyl □-hydroxy-3,3-dimethylbutyrate,methyl mandelate, methyl □-hydroxy-3-phenyl propionate, methyl□-hydroxyisobutyrate, ethyl □-hydroxyisobutyrate, propyl□-hydroxyisobutyrate, isopropyl □-hydroxyisobutyrate, butyl□-hydroxyisobutyrate, methyl □-hydroxy-2-ethylbutyrate, ethyl□-hydroxy-2-ethylbutyrate, propyl □-hydroxy-2-ethylbutyrate, ethyleneglycol diglycolate, ethylene glycol glycolate, propylene glycoldilactate, propylene glycol monolactate, and the like. Glycolide orlactide (which are cyclic dimers of structural formula (IV)), oligomericglycolates or lactates, and soluble low molecular weight polyesters ofglycolic or lactic acid or copolyesters containing glycolic acid orlactic acid monomers in their compositions are further examples of theglycolate of the formula (IV).

Examples of the glyoxalates of the formula (V) include methylglyoxalate, ethyl glyoxalate, propyl glyoxzalate, isopropyl glyoxalate,butyl glyoxalate, isobutyl glyoxalate, sec-butyl glyoxalate, tert-butylglyoxalate, 2-ethylhexyl glyoxalate, phenyl glyoxalate, ethylene glycolmono- and diglyoxalate and mixtures thereof, methyl pyruvate, ethylpyruvate, isopropyl pyruvate, butyl pyruvate, isobutyl pyruvate,sec-butyl pyruvate, tert-butyl pyruvate, phenyl pyruvate, propyleneglycol mono- and dipyruvate and mixtures thereof, methylphenylglyoxalate, ethyl phenylglyoxalate, propyl phenylglyoxalate,isopropyl phenylglyoxalate, butyl phenylglyoxalate, isobutylphenylglyoxalate, sec-butyl phenylglyoxalate, tert-butylphenylglyoxalate, mono- and diesters of phenylglyoxalic acid with2-phenyl, and the like.

Examples of the oxalates of the formula (V) include dimethyl oxalate,diethyl oxalate, dipropyl oxalate, diisopropyl oxalate, dibutyl oxalate,diisobutyl oxalate, di-sec-butyl oxalate, bis(2-ethyl hexyl)oxalate,di-tert-butyl oxalate, diphenyl oxalate, dicyclohexyl oxalate, dibenzyloxalate, and low molecular weight oligomeric esters produced fromcondensation of oxalates, such as those above and varying amounts ofethylene glycol.

The 1,2-diol that can be produced using the process of the invention isnot particularly limiting. Examples of such diols include1,2-ethanediol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol,1,2-hexanediol, 2-methyl-1,2-propanediol, 1-phenyl-1,2-ethanediol, and3-phenyl-1,2-propanediol.

The anhydrous alcohols useful in the present invention typically contain2 to 12 carbon atoms. The alcohols may be linear, branched, or cyclic.Specific examples of suitable alcohols include ethanol, n-propanol,isopropanol, n-butanol, isobutanol, etc.

In various embodiments, the alcohol is ethanol. Anhydrous ethanol iscommercially available in various grades, such as 200 proof, ≥99% ofethanol by volume, ≥99.5% of ethanol by volume, <1% of water by volume,<0.5% of water by volume, or <0.005% of water by volume. Any of thesegrades may be used in the TH reaction.

Preferably, the reaction mixture contains less than 1 wt %, less than0.5 wt %, less than 0.4 wt %, less than 0.3 wt %, less than 0.2 wt %,less than 0.1 wt %, less than 0.05 wt %, less than 0.01 wt %, less than0.005 wt %, or less than 0.001 wt % of water, based on the total weightof the reaction mixture. In one embodiment, the TH reaction is carriedout in the absence of water.

The contacting step/TH reaction is preferably carried out using excessalcohol. For example, the molar ratio of the C2-C12 alcohol to the estercan be from 2:1 to 100:1, and all ranges in between including 2:1 to50:1 and 10:1 to 30:1.

In one embodiment, the ester comprises methyl glycolate, the C2-C12alcohol comprises ethanol, and the 1,2-diol comprises ethylene glycol.

In another embodiment, the ester comprises dimethyl oxalate, the C2-C12alcohol comprises ethanol, and the 1,2-diol comprises ethylene glycol.

In yet another embodiment, the ester comprises methyl lactate, theC2-C12 alcohol comprises ethanol, and the 1,2-diol comprises propyleneglycol.

The transfer hydrogenation process according to the invention canproduce valuable by-products, such as ethyl acetate. The ethyl acetatemay be isolated and purified by conventional methods and sold as acommodity chemical. Alternatively, the ethyl acetate can be hydrogenatedat mild conditions and recycled as the reducing agent in an ambientpressure EG process.

The catalyst of the formula (I) may be prepared in multiple ways. Forexample, the catalyst may be formed in situ by introducing apre-catalyst of the formulas (IIa) or (IIb):

into the reactor and exposing the pre-catalyst to heat, an acid, a base,or combinations thereof to form the catalyst of the formula (I).

R1, R2, R3, R4, R5, R6, E, and L in the formulas (IIa) or (lib) are asdefined in formula (I).

Z in the formula (IIa) is R7 or X.

R7 is hydrogen or an alkyl or aryl group.

X is [BH4]- or a halide.

L2 in the formula (IIb) is a neutral ligand.

The alkyl or aryl group represented by R7 may contain from 1 to 12carbon atoms.

The halides represented by X include chloride, bromide, and iodide. Inone embodiment, X is chloride or bromide.

Examples of the neutral ligand L2 include an ether compound, an amidecompound, a nitrile compound, and an N-containing heterocyclic compound.

In one embodiment, when X is a halide, the pre-catalyst is exposed to abase and optionally to heat to generate the catalyst.

In another embodiment, when X is [BH4]-, the pre-catalyst is exposed toheat, but optionally in the absence of a base, to generate the catalyst.

Unless the context clearly suggests otherwise, as used herein, theexpression “in the absence of” means that the referenced component isnot added from an external source (i.e., one that is independent of thereactants) or, if added, is not added in an amount that affects the THreaction to an appreciable extent, for example, an amount that canchange the yield of the corresponding alcohol by more than 10%, by morethan 5%, by more than 1%, by more than 0.5%, or by more than 0.1%.

In various embodiments, the pre-catalyst of the formula (IIa) has theformula (1a):

where ^(i)Pr represents an isopropyl group.

In various embodiments, the pre-catalyst of the formula (IIb) has theformula (1b):

where ^(i)Pr represents an isopropyl group.

Alternatively, the catalyst of the formula (I) may be formed in situ bythe steps of:

(a) introducing (i) an iron salt or an iron complex comprising theneutral ligand (L), (ii) a ligand of the formula (III):

and (iii) optionally the neutral ligand (L) into the reactor to form apre-catalyst mixture; and

(b) optionally exposing the pre-catalyst mixture to heat, an acid, abase, or combinations thereof to form the catalyst of the formula (I).

R1, R2, R3, R4, R5, R6, and E in the formula (III) are as defined informula (I).

Examples of iron salts suitable for making the catalyst of the formula(I) include [Fe(H2O)6](BF4)2, Fe(CO)5, FeCl2, FeBr2, FeI2, [Fe3(CO)12],Fe(NO3)2, FeSO4, and the like.

Iron complexes comprising the neutral ligand (L) may be made by methodsknown in the art and/or are commercially available.

Ligands of the formula (III) may be made by methods known in the artand/or are commercially available.

The heat employed for generating the catalyst is not particularlylimiting. It may be the same as the heat used for the TH reaction. Forexample, the pre-catalyst or pre-catalyst mixture may be exposed toelevated temperatures, such as from 40 to 200° C., 40 to 160° C., 40 to150° C., 40 to 140° C., 40 to 130° C., 40 to 120° C., 40 to 100° C., 80to 160° C., 80 to 150° C., 80 to 140° C., 80 to 130° C., 80 to 120° C.,or 80 to 100° C., to form the catalyst.

The acid for forming the catalyst is not particularly limiting. Examplesof suitable acids include formic acid, HBF₄, HPF₆, HOSO₂CF₃, and thelike.

The base for forming the catalyst is not particularly limiting. Bothinorganic as well as organic bases may be used. Examples of suitableinorganic bases include Na, K, NaH, NaOH, KOH, CsOH, LiHCO₃, NaHCO₃,KHCO₃, CsHCO₃, Li₂CO₃, Na₂CO₃, K₂CO₃, Cs₂CO₃, and the like. Suitableorganic bases include metal alkoxides and nitrogen-containing compounds.Examples of suitable metal alkoxides include alkali-metal C₁-C₆alkoxides, such as LiOEt, NaOEt, KOEt, and KOt-Bu. In one embodiment,the base is sodium methoxide (NaOMe). In another embodiment, the base issodium ethoxide (NaOEt). Examples of nitrogen-containing bases includetrialkylamines, such as triethylamine.

Typically, a 1:1 molar equivalent of base to catalyst precursor is usedto generate the catalyst. More than a 1:1 molar equivalent ratio may beused, e.g., a 2:1 ratio of base to catalyst precursor. However, using alarge excess amount of base should be avoided, as it may suppress theformation of the 1,2-diol.

The conditions effective for forming the 1,2-diol include an elevatedtemperature. The temperature conducive for the TH reaction may range,for example, from 50 to 180° C., including all ranges in between, suchas from 75 to 100° C.

Advantageously, the TH reaction may be conducted at ambient pressure ornear ambient pressure. As noted, the process of the invention does notrequire a molecular hydrogen atmosphere. Therefore, preferably, thereaction is conducted in the absence of exogenous molecular hydrogen(H₂).

Preferably, the contacting step/TH reaction is carried out in theabsence of a base. Basic conditions during the reaction may tend tosuppress the formation of the 1,2-diol.

The TH reaction may be conducted in the presence or absence of asolvent. In one embodiment, the contacting step/TH reaction is conductedin the presence of a solvent. In another embodiment, the contactingstep/TH reaction is conducted in the absence of a solvent.

If desired, the TH reaction may be performed in common non-polarsolvents, such as aliphatic or aromatic hydrocarbons, or in slightlypolar, aprotic solvents, such as ethers. Examples of aliphatic solventsinclude pentanes and hexanes. Examples of aromatic solvents includebenzene, xylenes, toluene, and trimethylbenzenes. Examples of ethersinclude tetrahydrofuran, dioxane, diethyl ether, and polyethers.

In various embodiments, the reaction is conducted in benzene, xylene(s),mesitylene, or toluene at atmospheric pressure.

If used, the solvent may be added in amounts of 1:1 to 100:1 or 1:1 to20:1 (v/v), relative to the amount of the alcohol reactant.

The TH reaction can take place with catalyst loadings of 210 ppm (0.001mol %). For example, the reaction may be carried out with catalystloadings of 10 to 20,000 ppm (0.001 to 2 mol %), 10 to 15,000 ppm (0.001to 1.5 mol %), 10 to 10,000 ppm (0.001 to 1 mol %), 10 to 1,000 ppm(0.001 to 0.1 mol %), or 10 to 500 ppm (0.01 to 0.05 mol %).

The process of the invention may be carried out in a batch or continuousmode. The reaction product(s) may be separated by conventional means,and the catalyst may be recycled.

The process according to the invention can produce the 1,2-diol withyields of at least 50%, at least 60%, at least 70%, at least 80%, atleast 85%, at least 90%, at least 95%, or at least 99%. The reactiontimes in which these yields may be achieved include 20 hours or less, 18hours or less, 16 hours or less, 12 hours or less, 10 hours or less, or8 hour or less.

Low pressure hydrogenation of esters by TH allows for the production ofglycols, such as EG and PG, with advantages in capital and operatingcosts, safety of operation, and flexibility for smaller scale, batchoperations where high pressure facilities may not be readily available.

The present invention includes and expressly contemplates any and allcombinations of embodiments, features, characteristics, parameters,and/or ranges disclosed herein. That is, the invention may be defined byany combination of embodiments, features, characteristics, parameters,and/or ranges mentioned herein.

As used herein, the indefinite articles “a” and “an” mean one or more,unless the context clearly suggests otherwise. Similarly, the singularform of nouns includes their plural form, and vice versa, unless thecontext clearly suggests otherwise.

While attempts have been made to be precise, the numerical values andranges described herein should be considered to be approximations (evenwhen not qualified by the term “about”). These values and ranges mayvary from their stated numbers depending upon the desired propertiessought to be obtained by the present invention as well as the variationsresulting from the standard deviation found in the measuring techniques.Moreover, the ranges described herein are intended and specificallycontemplated to include all sub-ranges and values within the statedranges. For example, a range of 50 to 100 is intended to describe andinclude all values within the range including sub-ranges such as 60 to90 and 70 to 80.

The content of all documents cited herein, including patents as well asnon-patent literature, is hereby incorporated by reference in theirentirety. To the extent that any incorporated subject matter contradictswith any disclosure herein, the disclosure herein shall take precedenceover the incorporated content.

This invention can be further illustrated by the following examples ofpreferred embodiments thereof, although it will be understood that theseexamples are included merely for purposes of illustration and are notintended to limit the scope of the invention unless otherwisespecifically indicated.

EXAMPLES General Experimental Information

Unless otherwise noted, all the organometallic compounds were preparedand handled under a nitrogen atmosphere using standard Schlenk andglovebox techniques. Anhydrous EtOH (200 proof) and toluene werepurchased from Sigma Aldrich and stored with 4 Å molecular sieves. BothEtOH and toluene were freshly distilled prior to use. Dimethyl1,4-cyclohexanedicarboxylate (DMCD, a mixture of cis and transisomers, >90% purity) was purchased from Alfa Aesar and used withoutfurther purification. Compounds 1a-c have been previously reported inthe literature. They were synthesized according to procedures that areslightly modified from the literature procedures.

Example 1

Synthesis of 1a [(^(iPr)PNHP)Fe(H)(CO)(Br)]

In a glovebox, under a nitrogen atmosphere, a 200-mL oven-dried Schlenkflask was charged with complex [^(iPr)PNHP]FeBr₂(CO) (850 mg, 1.545mmol), NaBH₄ (60 mg, 1.545 mmol, 98% purity), and 100 mL of dry EtOH.The resulting yellow solution was stirred for 18 hours at roomtemperature and filtered through Celite. The filtrate was evaporated todryness to obtain pure 1a (86% isolated yield). The ¹H and ³¹P{¹H} NMRspectra of 1a agreed well with the reported values (see S. Chakrabortyet al., J. Am. Chem. Soc. 2014, 136, 7869).

Example 2

Modified Synthesis of 1 b [(^(iPr)PNHP)Fe(H)(CO)(HBH₃)]

In a glovebox, under a nitrogen atmosphere, a 200-mL oven-dried Schlenkflask was charged with complex [^(iPr)PNHP]FeBr₂(CO) (850 mg, 1.545mmol), NaBH₄ (131 mg, 3.399 mmol, 98% purity), and 100 mL of dry EtOH.The resulting yellow solution was stirred for 18 hours at roomtemperature and filtered through Celite. The filtrate was evaporated todryness to obtain pure 1b (84% isolated yield). The ¹H and ³¹P{¹H} NMRspectra of 1b agreed well with the reported values (see S. Chakrabortyet al., J. Am. Chem. Soc. 2014, 136, 7869).

Example 3

Modified Synthesis of 1c [(^(iPr)PNP)Fe(H)(CO)]

In a glovebox, under a nitrogen atmosphere, a 200-mL oven-dried Schlenkflask was charged with complex 1b (500 mg, 1.06 mmol), NaOtBu (106 mg,1.07 mmol, 97% purity), and 60 mL of dry THF. Immediately, a deep redsolution resulted, which was stirred for an additional 30 minutes atroom temperature. After that, the solvent was removed under vacuum, andthe desired product was extracted into pentane and filtered through aplug of Celite to remove NaBr. The resulting filtrate was evaporatedunder vacuum to afford pure 1c (76% isolated yield). The ¹H and ³¹P{¹H}NMR spectra of 1c agreed well with the reported values (see S.Chakaraborty et al., J. Am. Chem. Soc. 2014, 136, 8564).

Example 4 Iron-Catalyzed Transfer Hydrogenation of Methyl Glycolate inthe Presence of EtOH

Under an inert atmosphere, an oven-dried 200-mL thick-wall Schlenk tubeequipped with a stir-bar was charged with compound 1c (0.1 mmol), methylglycolate (0.01 mol, 0.91 g, 98% pure), anhydrous EtOH (0.2 mol, 11.7mL), and 20 mL of anhydrous toluene. The resulting mixture was heated to100° C. for ˜16 h using an oil-bath. After ˜16 h, the brown coloredsolution was cooled to room temperature, volatiles were carefully ventedinside the hood, and the resulting liquid was analyzed by gaschromatography.

Under these conditions, 63.1% of methyl glycolate was converted to yield51.7% of ethylene glycol (EG). EtOAc, MeOH, and trace amounts of methylformate were also observed as other volatile byproducts.

Example 5 Iron-Catalyzed Transfer Hydrogenation of Dimethyl Oxalate inthe Presence of EtOH

Under an inert atmosphere, an oven-dried 200-mL thick-wall Schlenk tubeequipped with a stir-bar was charged with compound 1c (0.1 mmol),dimethyl oxalate (0.01 mol, 1.2 g, 99% pure), anhydrous EtOH (0.2 mol,11.7 mL), and 20 mL of anhydrous toluene. The resulting mixture washeated to 100° C. for ˜16 h using an oil-bath. After ˜16 h, the browncolored solution was cooled to room temperature, volatiles werecarefully vented inside the hood, and the resulting liquid was analyzedby gas chromatography.

Under these conditions, 47.8% of dimethyl oxalate was converted to yield18.4% of ethylene glycol (EG) and 23.5% of methyl glycolate. EtOAc,MeOH, and trace amounts of methyl formate were also observed as othervolatile byproducts.

In the specification, there have been disclosed certain embodiments ofthe invention and, although specific terms are employed, they are usedin a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims.

We claim:
 1. A process for preparing a 1,2-diol, the process comprisingcontacting an ester of the formulas (IV) or (V):

with an anhydrous C₂-C₁₂ alcohol in the presence of a catalyst of theformula (I):

in a reactor at ambient pressure and elevated temperature for a timesufficient to form a 1,2-diol, wherein R¹ and R² are each independentlyan alkyl, aryl, alkoxy, aryloxy, dialkylamido, diarylamido, oralkylarylamido group having 1 to 12 carbon atoms; R³ and R⁴ are eachindependently an alkyl or aryl group having 1 to 12 carbon atoms, if Eis nitrogen; R³ and R⁴ are each independently an alkyl, aryl, alkoxy,aryloxy, dialkylamido, diarylamido, or alkylarylamido group having 1 to12 carbon atoms, if E is phosphorus; R¹, R², and P may be connected toform a 5 or 6-membered heterocyclic ring; R³, R⁴, and E may be connectedto form a 5 or 6-membered heterocyclic ring; R⁵ and R⁶ are eachindependently a C₁-C₆ alkylene or arylene group; E is phosphorus ornitrogen; L is a neutral ligand; R¹⁰ is an alkyl or aryl group having 1to 20 carbon atoms; R¹¹ is each independently hydrogen, or an alkyl oraryl group having 1 to 20 carbon atoms; and R¹² is hydrogen, or analkyl, aryl, alkoxy, or aryloxy group having 1 to 20 carbon atoms. 2.The process according to claim 1, wherein the catalyst is formed byintroducing a pre-catalyst of the formulas (IIa) or (lib):

into the reactor and exposing the pre-catalyst to heat, an acid, a base,or combinations thereof; and wherein R¹, R², R³, R⁴, R⁵, R⁶, E, and Lare as defined in formula (I); Z is R⁷ or X; R⁷ is hydrogen or an alkylor aryl group; X is [BH₄]⁻ or a halide; and L² is a neutral ligand. 3.The process according to claim 1, wherein the catalyst is formed by: (a)introducing (i) an iron salt or an iron complex comprising the neutralligand (L), (ii) a ligand of the formula (III):

and (iii) optionally the neutral ligand (L) into the reactor to form apre-catalyst mixture; and (b) optionally exposing the pre-catalystmixture to heat, an acid, a base, or combinations thereof; wherein R¹,R², R³, R⁴, R⁵, R⁶, and E are as defined in formula (I).
 4. The processaccording to claim 1, wherein one or more of R¹, R², R³, and R⁴ aresubstituted with one or more groups selected from ethers and amides. 5.The process according to claim 1, wherein R¹, R², R³, and R⁴ are eachindependently a methyl, ethyl, propyl, isopropyl, butyl, pentyl,isopentyl, cyclopentyl, hexyl, cyclohexyl, or phenyl group.
 6. Theprocess according to claim 5, wherein each of R¹, R², R³, and R⁴ isisopropyl.
 7. The process according to claim 5, wherein each of R¹, R²,R³, and R⁴ is phenyl.
 8. The process according to claim 1, wherein eachof R⁵ and R⁶ is —(CH₂CH₂)—.
 9. The process according to claim 1, whereinE is phosphorus.
 10. The process according to claim 1, wherein L iscarbon monoxide, a phosphine, an amine, a nitrile, or an N-containingheterocyclic ligand.
 11. The process according to claim 2, wherein L² isan ether, an amide, a nitrile, or an N-containing heterocyclic ligand.12. The process according to claim 1, wherein the contacting step isconducted at a temperature of 50 to 180° C.
 13. The process according toclaim 1, wherein the contacting step is conducted at a temperature of 75to 100° C.
 14. The process according to claim 1, wherein the contactingstep is conducted in the absence of exogenous molecular hydrogen (H₂).15. The process according to claim 1, wherein the contacting step isconducted in the presence of a solvent.
 16. The process according toclaim 2, wherein the base is a metal alkoxide or a nitrogen-containingcompound.
 17. The process according to claim 16, wherein the base issodium methoxide, sodium ethoxide, or triethylamine.
 18. The processaccording to claim 1, wherein the molar ratio of the C₂-C₁₂ alcohol tothe ester ranges from 2:1 to 100:1.
 19. The process according to claim1, wherein the C₂-C₁₂ alcohol comprises ethanol, isopropanol, orisobutanol.
 20. The process according to claim 1, wherein the ester is acompound of the formula (IV).
 21. The process according to claim 1,wherein the ester is a compound of the formula (V).
 22. The processaccording to claim 20, wherein the ester comprises methyl glycolate, theC₂-C₁₂ alcohol comprises ethanol, and the 1,2-diol comprises ethyleneglycol.
 23. The process according to claim 20, wherein the estercomprises methyl lactate, the C₂-C₁₂ alcohol comprises ethanol, and the1,2-diol comprises propylene glycol.
 24. The process according to claim21, wherein the ester comprises dimethyl oxalate, the C₂-C₁₂ alcoholcomprises ethanol, and the 1,2-diol comprises ethylene glycol.